1. Field of the Invention
The present invention relates to a flat display device, and more particularly, to an organic electroluminescent display device having an organic thin film transistor in which a gate insulating layer uses an organic insulating layer, and a capacitor dielectric uses an inorganic insulating layer, and a method of fabricating the same.
2. Description of the Related Technology
An organic thin film transistor (OTFT) has been actively researched as a driving element of a next-generation display device. The OTFT uses an organic layer as a semiconductor layer instead of a silicon layer. The OTFT can be classified into a low molecular OTFT such as oligothiophene, pentacene, etc., and a polymer OTFT such as polythiophene, etc., in accordance with the composition of the organic layer.
Korean Patent Publication No. 2004-0028010 discloses an OTFT using an organic insulating layer as a gate insulating layer. It uses an optical orientation element to an organic polymer layer for a gate insulating layer to increase the orientation of an organic active layer. Further, Korean Patent Publication No. 2004-0049110 discloses a thin film transistor having a trench formed in an insulating layer which in turn is formed on a gate insulating layer and an organic semiconductor layer formed in the trench.
An organic electroluminescent display device using such an OTFT as a switching element includes at least two OTFTs, for example, one switching OTFT and one drive OTFT, one capacitor, and an organic electroluminescent element having an organic layer between upper and lower electrodes.
FIG. 1 is a sectional view illustrating a conventional organic electroluminescent display device having an OTFT.
Referring to FIG. 1, a drive OTFT for driving (i.e., providing current to) the organic electroluminescent element, a capacitor, and an organic electroluminescent element are formed on a substrate 100. The OTFT includes a gate electrode 111, a source electrode 141 and a drain electrode 145 formed on a gate insulating layer 121, and a semiconductor layer 150 formed on the substrate 100. The capacitor includes a lower electrode 117 formed on the substrate 100, and an upper electrode 147, which is connected to the source electrode 141 of the OTFT. A capacitor dielectric 125 is interposed between the lower electrode 117 and the upper electrode 147.
The organic electroluminescent element includes i) an anode electrode 170 as a lower electrode, which is formed on a protecting layer 160 and connected to the drain electrode 145 through a via hole 165, ii) an organic layer 190, and iii) a cathode electrode 195 as an upper electrode formed on the overall surface of the substrate 100. A pixel definition layer 180 is formed on the protecting layer 160 and has an opening 185 exposing a portion of the anode electrode 170. An organic layer 190 is formed on the anode electrode 170 exposed by the opening 185.
The organic electroluminescent display device having the OTFT described as above has an insulating layer 120 formed on the gate electrode 111, the lower electrode 117 of the capacitor, and the substrate 100. The insulating layer 120 serves as a gate insulating layer of the thin film transistor (TFT) as well as a dielectric layer of the capacitor. That is, a portion 121 of the insulating layer 120, which is interposed between the gate electrode 111 of the TFT and a semiconductor layer 150, serves as a gate insulating layer of the TFT. A portion 125 of the insulating layer 120, which is interposed between the upper electrode 147 and the lower electrode 117 of the capacitor, serves as a dielectric layer of the capacitor.
The OTFT structured as above is employed in a flexible flat display device and the like, and an organic material is used as a gate insulating layer. Generally, an organic material such as PVA, etc., has a dielectric constant as low as about 4.9. Thus, if the organic material is used as a gate insulating layer, the layer must be thick in order to maintain the requisite insulation.
However, where that the gate insulating layer is too thick, the capacitor which uses a portion of the layer as a capacitor dielectric may have deficient capacitance.
On the other hand, if the organic gate insulating layer is too thin, the capacitor may function properly, but the desired level of insulation for the gate may not be adequate.